The present invention relates to porous silicon, particularly to the fabrication and use of porous silicon structures, and more particularly to the fabrication and use of structures which benefit from a high surface area or a specific pore size of porous silicon.
In recent years crystalline silicon micromachining technology has enabled the development of numerous miniaturized devices such as chemical reaction chambers, micro-actuators, micro-grippers, microvalves, etc. Also, it has been recently established that miniaturized porous silicon structures can be fabricated using micromachining technology. P. C. Searson, "Porous Silicon Membranes", Appl. Phys. Lett. 59 (7), 12 Aug. 1991; and S. Ottow et al., "Processing Of Three-Dimensional Microstructures Using Microporous n-Type Silicon", J. Electrochem. Soc., Vol. 143, No. 1, January 1996. Porous silicon is a material that is fabricated from crystalline silicon. Very small pores (nm-.mu.m diameters) can be introduced with a relatively high degree of uniformity and control. The present invention is based on the recognition that porous silicon compared to crystalline silicon provides a means to significantly increase the surface area of a silicon device or provide specific pore size arrays, while maintaining capability of modifying them with integrated circuit (IC) processes, or by micromachining processes. For example, a doped porous silicon membrane with appropriate pore diameter, which can be fabricated using known IC processes, could be used to provide a resistive heater and/or it could contain electrodes or arrays of electrodes to control the flow of electrically charged chemicals through the pores (such as in electrophoresis). A heated porous silicon membrane could be used to control the viscosity or surface tension of a fluid, thereby making it a selectable/active filtering or valving device. The high surface area of such a porous silicon heater is expected to increase the internal pressure of a liquid/vapor filled closed cavity, thereby increasing the force generated for membrane deflection in a microfabricated thermopneumatic valve. As well, the high surface area pores of porous silicon could be coated with specific coatings for the adsorption/desorption of liquids or gasses, thereby creating a chemical species concentrator. The specific pore size of the porous silicon enables its use as chemical/biological filters and thermally-activated flow devices with active or adjacent surfaces such as electrodes or heaters.